The present invention relates to a photosensitive imaging system of the type which employs microcapsules containing a photosensitive composition in the internal phase. More particularly, it relates to a photosensitive system wherein the developer sheet has reduced tack.
Photosensitive imaging systems employing microencapsulated radiation sensitive compositions are the subject of commonly assigned U.S. Pat. Nos. 4,399,209 and 4,416,966 as well as co-pending U.S. patent application Ser. No. 320,643 filed Jan. 18, 1982. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photohardenable or photosoftenable photosensitive composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photosensitive composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color precursor. The exposure image-wise hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is subjected to a uniform rupturing force by passing the sheet through the nip between a pair of pressure rollers. U.S. Pat. No. 4,399,209 discloses a transfer system in which the imaging sheet is assembled with a developer sheet prior to being subjected to the rupturing force. Upon passing through the pressure rollers in contact with the developer sheet, the microcapsules image-wise rupture and release the internal phase whereupon the color precursor migrates to the developer sheet where it reacts with a dry developer and forms a color image. The imaging system can be designed to reproduce monochromatic or polychromatic full color images.
Commonly assigned U.S. Pat. No. 4,554,235 provides an imaging system which is useful in producing high gloss images and is characterized in that the reactive surface of the developer sheet is overcoated with a discontinuous layer of a thermoplastic polymeric pigment. Images are formed as described in U.S. Pat. Nos. 4,399,209 and 4,416,966. After development, the developer sheet is heated to coalesce the thermoplastic pigment and form a thin uniform layer which imparts gloss to the image.
Commonly assigned U.S. application Ser. No. 905,727 filed Sept. 9, 1986 describes a developer sheet which is used to obtain glossy images which obviates the need for the layer of thermoplastic pigment described in the aforementioned patent. This sheet carries a layer of a finely divided thermoplastic developer resin such as a novolak. The developer layer is obtained from a dispersion of the resin in water. After transfer, the color precursor reacts with the developer and the sheet is heated to melt the developer resin. Thereupon the finely divided particles coalesce into a thin, uniform film. By controlling the heating conditions, the degree of gloss can be controlled to give copies with finishes ranging from matte to high gloss.
While the developer sheets described in the aforementioned application are extremely advantageous, several problems have been encountered in designing sheets for commercial use. One problem is that the developer resins are undesirably tacky. This tackiness results in the developer sheets being difficult to handle. For example, when the developer sheets are stacked, there is a tendency for them to stick to one another. Also, when the developer sheets are fed into the processing equipment, the developer resin may soil paper handling rollers. Also, after the imaged developer sheets are fused, the tendency to stick increases. During any phase in the life of a developer sheet, if the tackiness problem is severe enough, the entire developer layer can be stripped from the developer sheet.
In the carbonless paper art, developer sheets prepared from novolak resins have been modified as described below to improve their performance. In the past, finely divided developer resins have been obtained by grinding a mixture of a developer such as a phenolic resin or a zinc salicylate with another resin and clay to produce a resin grind which is applied to the surface of the developer sheet. For example, U.S. Pat. No. 3,924,027 discloses a composition including a zinc salicylate, polystyrene, and kaolin.
One approach to improving the handling characteristics of developer sheets is to incorporate a stilt material such as starch particles as disclosed in commonly assigned U.S. Pat. Nos. 4,399,209 and 4,440,846.
U.S. Pat. No. 4,470,058 teaches a two-coat record sheet for printing by xerographic methods wherein the nonreactive topcoat eliminates the accumulation of contaminants on the fuser roll of copier/duplicators. The record sheet has a base coat comprising a phenol-formaldehyde novolak resin and a topcoat comprising a nonreactive pigment material. The reference discloses that the preferred nonreactive pigments are such materials as kaolin clay, calcium carbonate and calcined kaolin clay.
Commonly assigned U.S. Pat. No. 3,617,410 discloses a record sheet having improved resistance to smudging wherein the base sheet is coated with a composition comprising a finely powdered, oil-soluble, acid-reactant, polymeric material such as a novolak resin, a substantially nonreactive pigment such as talc, clay, colloidal silica, etc. and a binder.
U.S. Pat. No. 4,422,670 discloses a developing sheet designed for high speed printing so as to rapidly set the printing ink containing a developing layer consisting of a color developing agent, calcium carbonate and styrene-butadiene copolymer latex or modified styrene-butadiene copolymer latex of an average particle size of less than 0.08 microns. The latex reduces pigment pick off. The combination is taught to result in an improvement in the water resistance, mark formation ability and printability of the pressure-sensitive recording paper.